1. Field of the Invention
The present invention relates to bonding of materials at room temperature and, in particular, to bonding of processed semiconductor materials, such as integrated circuit or device substrates, having activated surfaces to achieve high bonding strength adequate for subsequent fabrication and/or a desired application.
2. Background of the Invention
Direct room temperature bonding generally produces weak van der Waals or hydrogen bonding. Annealing is typically required to convert the weak bond to a stronger chemical bond such as a covalent bond. Other wafer bonding techniques including anodic and fusion typically require the application of voltage, pressure and/or annealing at elevated temperature to achieve a sufficient bond strength for subsequent fabrication and/or the desired application. The need to apply voltage, pressure or heat has significantly limited wafer bonding applications because these parameters can damage the materials being wafer bonded, give rise to internal stress and introduce undesirable changes in the devices or materials being bonded. Achieving a strong bond at low temperatures is also critical for bonding of thermally mismatched or thermally sensitive wafers including processed device wafers.
Ultra high vacuum (UHV) bonding is one of the approaches to achieve a low or room temperature strong bond. However, the bonding wafers still have to be pre-annealed at high temperatures, for instance >600° C. for silicon and 500° C. for GaAs, before cooling down to low or room temperature for bonding. Furthermore, the UHV approach does not generally work on commonly used materials, for example, in SiO2. It is further also expensive and inefficient.
Adhesive layers can also be used to bond device wafers to a variety of substrates and to transfer device layers at low temperatures. However, thermal and chemical instability, interface bubbles, stress and the inhomogeneous nature of adhesive layers prevent its wide application. It is thus highly desirable to achieve a strong bond at room temperature by bonding wafers in ambient without any adhesive, external pressure or applied electric field.
Low vacuum bonding has been explored as a more convenient alternative to UHV bonding but a bonding energy comparable to the bulk silicon fracture energy using bonded bare silicon wafer pairs has only be achieved after annealing at ˜150° C. For oxide covered silicon wafer pairs annealing at ˜300° C. is required to obtain a high bond energy. It has not been possible to obtain high bonding energies in bonded material using low vacuum bonding at room temperature.
A gas plasma treatment prior to bonding in ambient is known to enhance the bonding energy of bonded silicon pairs at low or room temperature. See, for example, J. de Physique, 49(C4), 79 (1988); G. G. Goetz, Proc. of 1st Intl. Symp. on Semicond. Wafer Bonding: Science, Technol. and Applications, The Electrochem. Soc., 92-7, 65 (1992); S. Farrens et al., J. Electroch. Soc., 142, 3950 (1995) and Amirffeiz et al, Abstracts of 5th Intl. Symp. on Semi. Wafer Bonding: Science, Tech. and Appl., The Electrochemical Society, 99-2, Abstract No. 963 (1999). Although these treatments have increased the bond energy obtainable at low or room temperature, they have only been demonstrated with planar silicon wafers or with silicon wafers using a plasma process that results in oxide being grown on the wafers during the plasma process. Moreover, these treatments have only been used to increase the bond energy by charging or damaging the surface. Furthermore, these treatments have not been used or shown to be applicable to deposited dielectrics or other materials.
Obtaining low or room temperature bonding with a method that is not only applicable to planar silicon and grown oxide surfaces but further to deposited materials and non-planar surfaces with planarized deposited materials will allow generic materials, including processed semiconductor wafers, to be bonded with minimal damage for manufacturing purposes. Such a method based on etching and chemical bonding is described herein.